An electronic component is mounted on a printed circuit board or the like. In general, the mounting of the electronic component is performed by a so-called reflow method in which, after temporarily jointing the printed circuit board or the like and the electronic component with a solder ball for semiconductor mounting (hereinafter, referred to as “solder ball”) and a flux, the entire printed circuit board is heated so as to melt the above-described solder ball, and then, the board is cooled to room temperature so as to solidify the solder ball, thus ensuring a solid solder-joint (also referred to as simply joint).
When operating an electronic equipment incorporating a printed circuit board or the like, heat is generated inside the electronic equipment due to a current applied for the operation. Since the above-described solder ball connects materials having different thermal expansion coefficients, such as a silicon chip and a resin substrate, with each other, the solder ball is subjected to a thermal fatigue environment with the operation of the electronic equipment. As a result, cracks may propagate inside the solder ball, and receiving/sending of an electrical signal through the solder ball may be interfered. In general, long-term reliability of the solder ball under such a thermal fatigue environment is called a thermal fatigue property or a TCT (Thermal Cycling Test) property, and is regarded as the most important property required for the solder ball.
In recent years, a lead free solder alloy used as a connecting material in an electronic apparatus has been required so as to minimize a negative impact on the environment in disposal of the electronic apparatus, but it is not general to use pure Sn as a composition of the above-described solder ball. This is because, since pure Sn is extremely soft, cracks become easy to propagate during the process of a TCT test when examining the above-described thermal fatigue property, and long-term reliability of the solder ball degrade. Accordingly, as the composition of the solder ball, in general, in addition to a Sn—Ag eutectic composition (Ag: 3.5 mass %, Sn: balance), for example, a solder composition in which a small amount of Cu as a third element is added to the chemical composition of the above-described Sn—Ag eutectic composition is widely used as disclosed in Patent Literature 1 and Patent Literature 2.
That is, by increasing the concentration of Ag, an intermetallic compound called Ag3Sn is precipitated in a large number in the solder ball, and the solder ball is hardened by precipitation hardening, and thus, deformation of the solder ball against external force becomes smaller. Conventionally, it has been considered that, by increasing the concentration of Ag, even when a load resulting from thermal fatigue is generated, a displacement itself due to the thermal fatigue becomes smaller, and thus, propagation of cracks inside the solder ball can be retarded.
However, addition of about 3 mass % is not desirable because Ag is expensive, and if about 3 mass % of Ag is added, needle-like Ag3Sn is precipitated in a large amount in Sn, when the flux used during the above-described temporary jointing is vaporized by heat during the reflow, the gas is trapped in the needle-like Ag3Sn, and voids due to air bubbles are easy to be formed in the vicinity of the joint interface. In the case of a conventional solder ball with a ball size having a diameter of 180 [μm] or more, the area of a joint composed of the solder ball and an electrode was sufficiently large, and thus, even if the needle-like Ag3Sn was precipitated in the vicinity of the joint, a decrease in joint strength between the solder ball and the electrode did not become a problem, and it did not cause a negative impact on a thermal fatigue property.
However, the need for a solder ball having a diameter of less than 180 [μm] has been increased with an acceleration of miniaturizing/weight saving of recent portable electronic equipment, and in this case, the joint area of a solder-joint used in an electronic member is reduced, and therefore, suppression of voids has been more emphasized than ever before. Accordingly, a solder ball not using Ag, such as a Sn—Bi type alloy, has been proposed. Since Bi is dissolved in Sn, a needle-like precipitate such as the above-described Ag3Sn is not formed, and as a result, the above-described concern about voids is not raised even under the recent environment where the joint area is reduced.